(1) Field of the Invention
The present invention relates to a semiconductor device and manufacturing method thereof suitable for use in, for example, a gallium nitride based field effect transistor (GaNFET).
(2) Description of Related Art
Recently, development of a GaNFET (gallium nitride field effect transistor) is actively in progress, which makes use of an AlGaN/GaN hetero junction and the electron transit layer of which is GaN (gallium nitride).
Because of being a material having a wide band gap, a high breakdown electric field strength, and a large saturation electron speed, GaN is highly promising as a material of a high power output device for which a high voltage operation is required.
At present, for example, for a power device for a mobile phone base station, a high voltage operation of 40 V or higher is required and GaNFET is very promising.
For such GaNFET as a high power output device, it is necessary to reduce the contact resistivity of source electrode and drain electrode to increase power efficiency.
Hitherto, a GaNFET has a structure, for example as shown in FIG. 5, in which a GaN electron transit layer 2 and an AlxGa1-xN (0≦x≦1) electron supply layer 3 are formed in order on a substrate 1 and a gate electrode 5, a source electrode 6, and a drain electrode 7 are provided on the AlxGa1-xN (0≦x≦1) electron supply layer 3. In FIG. 5, symbol 4 denotes an SiN passivation film.
Then, as the source electrode 6 and the drain electrode 7 (that is, an ohmic electrode) provided on the AlxGa1-xN (0≦x≦1) electron supply layer 3, a Ti/Al electrode or a Ti/Al/Ni/Au electrode is mainly used.
As a result of the prior art search, published Japanese translation of PCT International Publication for Patent Application, No. 2005-509274 has been obtained.
By the way, as described above, in a GaNFET that uses the Ti/Al electrode or the Ti/Al/Ni/Au electrode as an ohmic electrode, the work function of Ti is 4.3 eV, therefore, there is a problem that a Schottky barrier is formed in between with an n-type III-V group nitride compound semiconductor.
When the Ti/Al electrode or the Ti/Al/Ni/Au electrode is used, a compound is generated at an interface between Ti and Al at the time of annealing to obtain the ohmic properties, and the melting point of the compound is not so high. Further, at the time of annealing, metal condenses, irregularities are formed, and the electrode surface becomes coarse [refer to FIG. 6(A) to FIG. 6(D)]. Furthermore, during operation at high temperatures, the Al atom, which is an electrode constituting element, moves (electromigration). These constitute factors that make the ohmic properties unstable at the time of device process or during operation at high temperatures.